Cosmologists are in the business of figuring out how and why we got here. But if you call their work unscientific, you're sure to set off a nerve.

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A recent story
in
Scientific American
managed to irk a whole bunch of these thinkers, so much that 33 of them (four of whom have Nobel prizes) signed a letter in response. Signatories included Steven Hawking, Leonard Susskind, Lisa Randall, and others who've written books and lectured to the world about how all we got here. At the center of the controversy is a popular theory that our universe inflated like a balloon right after the Big Bang. One group of scientists essentially said this theory wasn't science-which is like calling artist's work "not art," or a chef's "not food."

Physicists Anna Ijjas and Paul J. Steinhardt from Princeton, and Abraham Loeb from Harvard, wrote the initial story, titled "Pop Goes the Universe" and published it in
Scientific American
this past February. They challenged inflation, a collection of scientific models first proposed by Alan Guth around 1980 positing that immediately after the Big Bang, the universe expanded exponentially like an inflating balloon before settling down into the one we live in today. The authors stated that astronomical observations
have constrained inflation in a way that makes the remaining models in support of it unlikely. Instead, the authors' story bolstered a competing theory,
that the universe "bounced back" to where it is today after a collapse. (You can learn more about various theories for the formation of our universe, and this ongoing debate,
here
).

Steinhardt has written articles like this for
Scientific Americanbefore
. But a particular line in the most recent article's conclusion set people off: "...Inflationary cosmology, as we currently understand it, cannot be evaluated using the scientific method." In other words, Steinhardt argues, the dominant theory for the formation of our universe isn't testable, and thus, not real science.

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Inflation proposer and MIT professor Guth penned the response with physicists David Kaiser (MIT), Andrei Linde (Stanford), and Yasunori Nomura (UC Berkeley), then gathered famous signatories from across the physics world. The four disagreed with many of the statements in the
Scientific American
article, but "We were particularly in strong disagreement with the statements they made about the testability of inflation which we thought were completely without justification," Guth told Gizmodo. "We thought it was about time someone answered those objections."

The response letter,
also published
in
Scientific American,
sums up the physicists' complaints. Inflation isn't just one theory but many models, and no one thinks that all of these models are correct, it says. Some models have been ruled out, others haven't. Some of them have made correct predictions, like the average mass density of the universe, for example. The models have gotten more accurate over time with the help of more mathematical theories and observation. But the letter's main point is that while inflationary models have become the predominant way to explain the universe, they're still testable science, and could be disproven if the right evidence turned up.

"We judge theories by what predictions they make that we can test, not the ones they make that can't be tested."

When I called some of the signatories, they felt conflicted about the initial article, the response letter, or both. Renata Kallosh, a Stanford theoretical physicist, told Gizmodo in an email that she'd reviewed some of Steinhardt's papers, and they contained errors. MIT theoretical physicist and Nobel laureate Frank Wilczek told Gizmodo that the initial article was "very biased," a selective interpretation of inflation that bordered on slander. "It's kind of outrageous," he said. But still, he said he "was uncomfortable with the idea bringing some massive hammer of dissent down," and thought the new letter was possibly overkill.

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That's because the signers of the response letter-even Guth-agree that inflation isn't an established truth yet. "The model of inflation is incredibly unsatisfying," Lisa Randall, Harvard University physicist, told Gizmodo. "It's very hard to define exactly what is meant by inflation, and that is somewhat the source of disagreement here." Sean Carroll, theoretical physicist at Caltech, pointed me towards
his blog
, where he summed his (and my) opinions up:

We judge theories by what predictions they make that we can test, not the ones they make that can't be tested. It's absolutely true that there are important unanswered questions facing the inflationary paradigm. But the right response in that situation is to either work on trying to answer them, or switch to working on something else (which is a perfectly respectable option). It's not to claim that the questions are in principle unanswerable, and therefore the field has dropped out of the realm of science.

Inflation has made several predictions that have since been proven, the letter points out, by experiments like the Cosmic Background Explorer (COBE) and most recently the
Planck mission
. Those include its predictions of the shape of the universe (flat) and several properties of the cosmic microwave background, the furthest-traveling light in the universe from only a few hundred thousand years after the Big Bang.

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I reached out to Clara Moskowitz, the initial story's editor at
Scientific American
about how she felt (full disclosure,
I interned
at
SciAm
and Moskowitz was one of my editors). "This piece fits right into the wheelhouse of what
Scientific American
likes to publish and wants to do: host a healthy scientific debate," she responded. She was thrilled to see so many scientists talking about this, and is interested in hosting all sides of the story. She welcomed the response letter's authors to contribute articles as well.

And when I reached out to the story's initial authors to hear their thoughts on the letter, they pointed me towards their response and a
Frequently Asked Questions
page, where they wrote: "We urge all our colleagues to focus on solving the outstanding problems of inflation and keeping an open mind about other, yet unknown theories that avoid these problems altogether."

Which leaves us basically in the same place we started, with an imperfect paradigm that attempts to describe our universe, and scientists who want to explore other options. Moskowitz hopes discussions like this will move the field of physics forward, and Guth hopes that the letter has demonstrated that the community at large agrees inflationary models are real science. He thinks the buzz about all of this will die down quickly.